Airborne Gravity 2010 - Geoscience Australia

Airborne Gravity 2010
Performance of the Gedex High-Definition Airborne
Gravity Gradiometer
Summary
Kieran A. Carroll 1 , David Hatch 2 , and Brian Main 3
1 Gedex Inc., Toronto Canada (Kieran.Carroll@Gedex.ca)
2 Gedex Inc., Toronto Canada (David.Hatch@Gedex.ca)
3 Gedex Inc., Toronto Canada (Brian.Main@Gedex.ca)
In this paper, we provide a high-level description of the design for the Gedex High Definition Airborne
Gravity Gradiometer (HD-AGG) system, and show the performance achieved by several of the
elements of the system.
Introduction
The goal for Gedex is to develop a commercially viable airborne gravity gradiometer that achieves
post-process performance of 1 E/Hz 1/2 from 0.001 to 1 Hz. At a typical flying height and survey speed
this will produce gravity gradient survey data with a spatial resolution of 50-100 m and an RMS noise
of 1 E. To be productive in a survey aircraft, the system must produce data within this noise
specification while flying at a standard survey height of 80 m in turbulence conditions up to 1 m/s 2 .
The technical hurdles that must be overcome to achieve the noise specifications are formidable. The
instrument must be able to measure displacements of the proof masses to accuracy on the order of
10 -13 m which is about 1000 times smaller than the diameter of a hydrogen atom. As well, no gravity
gradiometer can distinguish between gravity gradient and angular velocity, requiring that the
instrument must be rotationally stabilized to within 0.06 deg/s. To achieve these and other extreme
specifications, Gedex is developing a highly sophisticated system comprised of a number of key subsystems.
The major components that make up the Gedex High-definition Airborne Gravity
Gradiometer system are as follows: gravity gradiometer instrument, cryostat, isolation mount, control
system, aircraft and post-processing software. Figure 1 shows the flight components of the system,
mounted in a Cessna 208. At the time of writing this paper (mid-2010), all subsystems have been built,
assemblies of some of these subsystems have been flight-tested, and preparations are underway for
flight-testing the complete, integrated system.
The gravity gradiometer instrument being developed for the HD-AGG is of the Cross-Component
Gradiometer type. This class of instrument has been independently demonstrated (Anstie et al., 2009)
to have acquired signal at sub-eotvos levels in a laboratory setting. The challenge faced by Gedex is
achieving the same level of performance on a moving platform. As it is impossible to build a
gradiometer that is totally insensitive to common mode accelerations, it is important to be able to
minimize the effects of these. One can remove the linear motion-induced noise that can be estimated
with independent accelerometers. As this compensation process cannot totally remove motion induced
noise, Gedex is also developing a motion isolation system to attenuate the aircraft accelerations that
the instrument experiences. The isolation mount will also protect the instrument from large
accelerations.
The end-to-end performance achieved by this system, in terms of the noise level of the gravity
gradiometry data reported after post-processing, is dependent on its various subsystems meeting their
own performance targets. Here we show the level of performance achieved by several of the HD-AGG
subsystems, in particular the gravity gradiometer instrument, the cryostat, the motion isolation mount,
and the post-processing software.
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